1. Field of the Invention
The present invention generally relates to an overlay accuracy measurement mark and a method of measuring an overlay accuracy between two selected patterns in a semiconductor device having two or more multi-patterns.
2. Brief Description of the Related Art
An overlay accuracy is known as an index showing an alignment state between multi-patterns, which is respectively formed during a step for forming one pattern and subsequent steps for forming another patterns. The index is measured using a pattern called as a box in box vernier pattern, which is formed on a scribe line separating a wafer to respective dies. The overlay accuracy is influenced by errors generated during the fabrication of a mask used in forming a pattern and of an overlay accuracy measurement system, and acts as an important factor for the alignment between two patterns.
FIG. 1 is a plan view schematically showing two patterns formed in a semiconductor substrate, and symbol H designates conductive region and symbols F and G designate contact regions. FIG. 2 shows a box in box vernier pattern of an overlay measurement mark for measuring an alignment state between two patterns. The overlay measurement mark is formed on a scribe line of a wafer using a plurality of masks which are identical to those used in the formation of patterns on a product die of the wafer.
Referring to FIGS. 1 and 2, in order to measure an overlay accuracy between two patterns shown in FIG. 1, a box shaped outer overlay mark 1 corresponding to the conductive region H of FIG. 1, is formed on the scribe line by a pattern mask used in the formation of the conductive region H. The mark 1 is formed concurrently with the formation of the conductive region and has a box shaped through hole in its central portion as viewed from top of the mark 1. Thereafter, a box-shaped inner overlay mark 2 corresponding to the contact regions F and G, is formed within the through hole of the outer overlay mark 1.
The overlay accuracy between the conductive region H and the contact regions F and G is measured from distances between two marks 1 and 2. Distances A and A' between the two marks 1 and 2 are measured as a degree of overlay in the X-axis, and the difference A-A' between the two distances is taken as an overlay error in the X-axis. Similarly, distances B and B' between the two marks 1 and 2 are measured as a degree of overlay in the Y-axis, and the difference B-B' between the two distances is taken as an overlay error in the Y-axis.
From the differences A-A' and B-B' obtained respectively in the X-direction and Y-direction, an alignment state between two patterns of FIG. 1 is determined. For example, if the differences A-A' and B-B' are both zero, it indicates that the two patterns H, and F and G are in exact alignment state. If the difference A-A'is positive, and B-B' is positive, it indicates that the contact regions F and G corresponding to the inner overlay mark 2 are shifted to positive X-direction and positive Y-direction by respective selected distances. If the differences A-A' is positive and B-B' is negative, it indicates that the contact regions F and G are shifted to positive X-direction and negative Y-direction by respective selected distances.
However, the above described overlay accuracy measurement mark can measure only the overlay error between selected two patterns. Therefore, to measure the overlay errors between any two selected patterns of three or more patterns which are formed on the product die, there should be additionally provided the same overlay accuracy measurement mark as that shown in FIG. 2. Also, the additional overlay measurement mark must be formed in a separate position from the previously formed overlay accuracy measurement mark on the scribe line. Moreover, several measurements are required depending on the number of patterns for which the overlay errors are to be measured. Furthermore, additional space is required because the additional overlay measurement marks must be placed in places other than the place where the measurement marks for measuring the overlay error between the other two patterns were formed.